The present invention concerns a part for a motor vehicle intended to come into contact with hydrocarbons.
It also concerns a method of making such a part impermeable [referred to herein as proofing it].
In the field of motor vehicles, more and more stringent standards are applicable concerning the emission of hydrocarbons over the entire vehicle.
These standards fix a maximum threshold, in grams per 24 hours, for the evaporation of volatile organic compounds per vehicle.
By way of example, the Euro 2000 standard defines an acceptable hydrocarbon emission threshold of 2 g/24 hours per vehicle.
For parts intended to come into contact with hydrocarbons, that is to say the parts situated close to the vehicle fuel tank, the maximum threshold corresponding to this Euro 2000 standard is 0.7 g/24 hours.
The acceptable hydrocarbon emission thresholds over the entire vehicle are distributed according to the performance of the different components.
At the present time, amongst all the parts for motor vehicles which are in contact with hydrocarbons, in the liquid or gaseous phase, there is a first type of plastic product, commonly referred to as the envelope: the parts are for example the tank filling head, the cap etc.
Products of a second type are made of rubber, such as the sleeves, the joints, or the pipes in the vehicle fuel system.
In order to meet the standards applicable with regard to the emission of hydrocarbons, all the parts of the envelope currently receive a fluoration treatment for making them impermeable. The rubber products for their part are produced from fluorocarbon rubber.
However, this fluoration treatment and the use of a fluorocarbon rubber give rise to a significant increase in the manufacturing cost of these parts.
The purpose of the present invention is to effect proofing of these parts in contact with hydrocarbons which is able to limit the emission of hydrocarbons to the maximum possible extent, but without giving rise to a high manufacturing cost for these parts.
According to the invention, a part for a motor vehicle, intended to come into contact with hydrocarbons, has a polytetrafluoroethylene (PTFE) coating adapted to make this part impermeable.
The Applicant found that such a polytetrafluoroethylene coating, traditionally used for its anti-adhesion properties, unexpectedly made it possible to achieve a seal at the parts thus coated as regards the emission of the volatile organic compounds coming from the hydrocarbons.
The proofing of these parts can thus be achieved on parts traditionally used in motor vehicles, without modification to the manufacturing tooling.
According to a preferred characteristic of the invention, the polytetrafluoroethylene coating covers a wall intended to come into contact with hydrocarbons.
Thus the PTFE coating forms on the part a xe2x80x9cbarrier layerxe2x80x9d in direct contact with the hydrocarbons in the liquid or gaseous phase, preventing any emission of gaseous components to the outside.
According to an advantageous characteristic of the invention, the thickness of the polytetrafluoroethylene coating is around a few tens of microns.
The longevity over time of these products coated with PTFE is appreciably greater than that of the products currently used, coated with a deposit of fluorine of a few microns, which rapidly erodes in time, notably because of the stirring of the fuel close to this deposit.
In practice, the part thus coated is made of plastic.
It may be used for components of the envelope, such as the tank caps or the fuel spouts.
Alternatively, the part thus coated can also be made of rubber, such as an elastomer.
The rubber is used in particular for producing pipes, sleeves or joints in the vehicle fuel system.
According to another aspect of the invention, a method of making impermeable a part for a motor vehicle intended to come into contact with hydrocarbons comprises a step of depositing a polytetrafluoroethylene coating.
The implementation of such a method does not require any particular precautions, unlike the methods currently used, using fluorine, which meet very strict regulations.
In practice, the polytetrafluoroethylene coating is deposited by spraying a liquid PTFE.
Such a method is similar to depositing paint with a gun in a suitable enclosure.
According to one advantageous characteristic of the invention, the deposited product comprises particles of PTFE, one or more solvents, and possibly a binding agent.
The use of solvents mixed with the PTFE particles makes it possible to spray this coating in liquid form onto the parts to be made impermeable.
In addition, adding a binding agent can help to stabilise this coating on the part to be treated, in particular depending on the material from which this part is produced.
According to an advantageous characteristic of the invention, the deposited product also comprises a pigment adapted to colour the PTFE coating.
It is thus possible, without complicating the manufacturing method, to colour a part coated with PTFE.
This colouring can in particular meet the requirements of the motor manufacturers who wish the joints visible outside the vehicle to be in a colour similar to that existing on the fuel pump nozzles in order to prevent any error during the filling of the vehicle tank.
Other features and advantages of the invention will also emerge from the following description.